The state of the art includes various components for hot runner injection molding systems, including nozzles and nozzle tips. Hot-runner nozzles are typically either a valve-gate style or a hot-tip style. In the valve-gate style, a separate stem moves inside the nozzle and tip acting as a valve to selectively start and stop the flow of resin through the nozzle. In the hot-tip style, a small gate area at the end of the tip freezes off to thereby stop the flow of resin through the nozzle.
An injection molding system using a hot-tip style nozzle typically has a cooled mold with a small circular gate opening in which the tip of the nozzle is inserted. The tip is typically conical with a tapered point or other suitable configuration. In operation, the tapered point is positioned in the circular gate to thereby form an annular opening through which molten plastic or other material is then transferred from the nozzle to the mold. When the mold is full, flow stops. In an ideal plastic molding cycle, the mold is typically cooled so that the plastic injected into it soon solidifies. As the liquid plastic in the mold cools it shrinks, which continues to allow plastic from the nozzle to move into the mold. This step is referred to as “packing”. The nozzle is typically heated so that the molten plastic contained within it remains liquid. The hot plastic moving through the gate area during packing keeps the gate area from freezing until all the plastic in the part has solidified. Eventually the gate freezes, the mold is opened, and the part is ejected, thereby breaking the small amount of frozen plastic at the gate area.
If the mold is opened before the gate has frozen, the plastic will string from the nozzle to the mold. This is known as a “gate stringing” and is unacceptable because the plastic string must be removed from the part in a subsequent operation, or the part scrapped. Waiting a long time for the gate to freeze is also unacceptable because it adds time to the molding cycle, which is desired to be as short as possible to optimize system productivity.
Many prior art nozzle tips function in essentially the same way, using the high thermal conductivity of the tip insert to conduct heat from the heated nozzle body to the gate area. The heat from the nozzle tip opens the gate at the beginning of the injection cycle and keeps it open during the injection process, and cooling from the mold cools and eventually freezes the gate after packing is complete. If the tip is not hot enough, the gate may not open and injection will not occur, or the gate will freeze too soon causing poor-quality parts. If too much heat is transferred to the tip, the gate will not freeze, resulting in stringing gates. Therefore, for any particular nozzle tip and resin there is an operating temperature window between the minimum temperature needed to get the gate open and keep it open as desired through the molding process, and the maximum temperature at which parts can be made without stringing gates. If the operating window is narrow, it may be difficult for molds with multiple cavities to consistently make good parts in all cavities because of the many variables associated with the injection molding process. One factor is assembly tolerance stack up that varies tip heights in the gate. For a conical tip, variations in tip height cause variations in the size of the annulus between the tip and the gate through which molten plastic flows. Another factor is variation in temperature of the resin from the nozzle to nozzle due to heat loss at various portions in the hot runner, or from flow imbalance in the hot runner. Furthermore, resins have melt flow characteristics and an optimum temperature range for processing that determines what processing parameters are used in the injection molding process. The flow characteristic for a resin inherently varies from batch to batch. To keep resin costs down and to preclude sorting resin by batch, molders often purchase resins in large quantities with a specification allowing a large range for flow characteristic. One batch of resin may run adequately for a given set of processing parameters, but the next batch, having a different flow characteristic, may not produce good parts using exactly the same process settings.
If the nozzle does not provide enough heat at the tip to keep the gate from freezing before the part is fully injected and packed, the part may have voids or other quality problems making it unacceptable. Heat is applied to the nozzle body by well-known techniques and is conducted to the nozzle tip. Thus, in the prior art, the tip material is generally made of high-conductivity material that promotes the flow of heat to the nozzle tip, such as a beryllium-copper alloy. It is important that the nozzle tip provide the right amount of heat at the gate area to keep the plastic in a liquid state as it flows through the gate, but also that it allows the plastic to freeze in a reasonable time when flow has stopped.
The tip must also resist corrosion, sustain compressive loads from injection pressures that may reach, e.g., from 26 ksi (179 MPa) to 40 ksi, (275 MPa) or higher at temperatures that may reach, e.g., 350° C., and resist wear when used with molding material such as plastics containing fillers, e.g., glass or other particulate materials. Since tips can wear out, it is desirable that they be easily replaceable. Thus, the nozzle tip must provide sufficient strength and resilience to sustain repeated uses under high temperature and pressure without failure. However, at these high pressures, existing nozzle tips exhibit an unacceptable failure rate. For example, beryllium-copper alloys are precipitation hardenable, and thus, can provide relatively high strength and wear-resistance, but low fatigue resistance. Accordingly, a great need exists for a nozzle tip that can adequately conduct heat, while possessing sufficient wear resistance and strength, particularly fatigue or endurance strength, to increase both the lifetime of the part and the maximum operating pressure. It is also desirable that tips be easily changed to process different materials. Other components of an injection molding assembly are subjected to similarly high stresses and temperatures, and thus, would also benefit from a component with high thermal conductivity and high strength.
U.S. Pat. No. 6,220,850 discloses a mold gate insert for a valve-gate style injection molding machine that is formed of two portions of differing materials. The material for the first portion is selected for its hardness and wear resistance, and non-precipitation hardening materials such as H13 tool steel, 420 ESR tool steel, and Vespel are disclosed as suitable materials. The material for the second portion is selected for its thermal conductivity, and beryllium copper alloy BeCu25 is disclosed as a suitable material. The first portion and second portion are joined together by physical means, such as press-fitting or swaging.
U.S. Patent Application Publication No. 2006/0196626 discloses the use of maraging steel alloys in injection molding machinery for providing better wear resistance and fatigue strength.
U.S. Pat. No. 4,451,974 discloses a nozzle for a valve-gate style injection molding machine that is formed of an outer conductive portion and a corrosion-resistant inner liner which are threaded together. The outer conductive portion is formed of a beryllium-copper alloy and the inner liner is formed of stainless steel.
U.S. Patent Application Publication No. 2005/0045746 discloses various components of a hot runner injection molding system, having a first portion and a second portion formed of different materials and fused together. The disclosure describes that the identities of the materials can be chosen for such material properties as thermal conductivity, wear resistance, strength, and resiliency.
U.S. Pat. No. 6,609,902 discloses a nozzle tip assembly that includes a nozzle tip retainer having high thermal conductivity, which holds a nozzle tip insert having lower thermal conductivity and high wear resistance. Materials disclosed for the conductive retainer include copper alloys and beryllium-copper alloys, and materials disclosed for the less conductive tip insert include stainless steel, tool steel, and carbide.
U.S. Pat. No. 6,164,954 discloses an injection nozzle that includes an inner portion formed of a material having high wear resistance and excellent thermal conductivity and an outer portion formed of a material having high pressure resistance and good thermal conductivity. The inner portion and the outer portion are joined together with a press-fit or interference fit to form the nozzle.
The present composite component and assembly are provided to address the problems discussed above and other problems, and to provide advantages and aspects not provided by prior components and assemblies of this type. A full discussion of the features and advantages of the present invention is provided in the following summary and detailed description, which proceeds with reference to the accompanying drawings.